1. Field of the Invention
The present invention relates to a sprocket for a bicycle.
2. Description of the Related Art
Recently, a sprocket having a radial bearing is widely used for the reason that adopting the radial bearing can enhance the precision of the bearing portion and make it superior in durability.
However, once the relative position between the inner race and the outer race of the radial bearing in an axial direction has a deviation, it will cause a defective rotation. As a certain amount of load may effect a deviation in the relative position between the inner race and the outer race in an axial direction, and result in a defective rotation, a load like that should be avoided. However, it is necessary to preload a certain amount of load with which swing is not felt in the least, even though a rider pedals a bicycle.
In a prior example as shown in FIG. 3, a left crank is mounted on a shaft with two fixing bolts disposed in the direction perpendicular to the shaft loosely. First, temporarily fixing bolts disposed in the axial direction of the shaft are fastened not to cause the crank to make a defective rotation and an axial swing. Next, the two fixing bolts disposed in the direction perpendicular to the shaft are fastened with equal forces, so that an axial force will not be generated with the left crank locked on the shaft. If the fastening forces of the two bolts are not equal, the two bolts and the left crank may be gradually loosening in use. Therefore, a beginner is required to perform a hard operation, due to the fact that it can be not adjusted after the left crank has been mounted.
In another prior example as shown in FIG. 4, fixing bolts disposed in a direction perpendicular to a shaft are fastened, such that a left crank can not be loosened any more even though a rider pedals a bicycle continuously for a long time. The left crank is secured through a friction to a shaft, or is secured at a locking position to a locking portion provided. At this time, an axial force is not applied to a bearing by the left crank, directly, but indirectly through an elastomer therebetween. As the force is blocked by the elastomer, it can be set without rendering a defective rotation and an axial swing. However, with the elastomer therebetween, when a large force is applied from the left crank side toward a right crank, the shaft will move temporarily toward the right crank to cause an adverse effect on a driving system. Hence, it can be not adjusted after the left crank has been mounted.
In a still further prior example as shown in FIG. 5, a shaft inserted into a left crank and secured therein passes through a sleeve 3, a left bearing and a right bearing, and then extends into a right crank. An adjusting nut is fastened to a screw thread portion matching with the adjusting nut without causing the shaft to swing. The screw thread portion for the adjusting nut is disposed at a base portion of the shaft for securing the right crank. Next, the portion for securing the right crank which is outside the adjusting nut, is inserted into the right crank, and then, fixing bolts, which are disposed in the direction perpendicular to the shaft are fastened, such that the right crank will not be loosened any more even though a rider pedals a bicycle continuously for a long time. At this time, the right crank abuts the end face of the adjusting nut. An axial force is applied to the adjusting nut by the right crank. Since a sleeve 1 and a sleeve 2 are disposed between the outer races and the inner races of the right and left radial bearings and the relative position of the outer races and the inner races forming the bearings remains unchanged, the force applied to the bearings will not cause the bearings to render a defective rotation. Because the right crank abuts the end face of the adjusting nut, the adjusting nut can not be loosened to change the adjustment. However, this method can not make an adjustment after mounting, either.